Current fault detector and circuit interrupter and packaging thereof

ABSTRACT

A power controller is positioned within a current path between the line side and the load side of an electrical circuit. The power controller closes the current path in the presence of a control supply and opens the current path in the absence of the control supply. A power supply electrically connected to the current path provides the control supply. A sensor system receives power from the power supply, monitors the current in the current path and outputs a sensor signal indicative of a current condition within the current path. A logic controller also receives power from the power supply, receives the sensor signal and removes the control supply from the power controller when the sensor signal does not satisfy an established criteria. The sensor system may include one or both of an imbalance sensor for monitoring the current balance among two or more electrical lines and over-current sensors for monitoring current in individual lines.

RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 09/954,474, filedSep. 14, 2001 which is a continuation-in-part of application Ser. No.09/775,337, filed Feb. 1, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to electrical control systems, and morespecifically to an aircraft electrical control system that monitors thecurrent condition in a current path and interrupts the current path upondetection of a current fault.

2. Description of the Related Art

In the electro-mechanical arts, current imbalances are indicative ofserious problems that can lead to disastrous results, such as arcingwithin fuel pumps. Since fuel pumps are often housed within a fuelvessel to directly pump fuel out of the vessel, arcing within a fuelpump can lead to an explosion of fuel-air mixture and a subsequentbreach of the fuel vessel, which can be catastrophic. In light of theseriousness of such an event, a device or methodology is needed whichcan suppress this type of arcing, as well as other associated problems.Presently, a common type of circuit protection device being utilized inaircraft is a thermal circuit breaker. However, arcing typically doesnot cause thermal circuit breakers to activate. Thus, there has been along-felt need for the function of current imbalance detection in anaircraft. One very important form of current imbalance is a ground faultin which current is flowing between a circuit or electrical device toground, when such current flow is not desired. In the prior art, groundfault detection has been addressed by a separate ground faultinterruption unit. However, such prior art systems have had limitations,including the necessity of rewiring the aircraft. In addition to therequirement to rewire the aircraft, additional space had to be found toaccommodate the ground fault interruption system.

One currently available ground fault interruption unit made by Autronics(model 2326-1) has been used in large commercial aircraft for thepurpose of ground fault protection for fuel pumps. The Autronics unitdetects a ground fault and outputs a signal indicative of a fault by useof a current transformer and acts by removing power to the fuel pumpcontrol relay.

There exists a need for an improved circuit protection device foraircraft. It would further be desirable for the circuit protectiondevice to be included within an existing device in the aircraft, or tobe packaged with an existing device, sharing the same connections toexisting electrical circuits, since space for avionics is limited in anyaircraft and adding wiring to accommodate a new device is verydifficult. The present invention addresses these and other concerns.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the invention is directed to devices andmethod for monitoring the current conditions within a current pathbetween the line side and load side of an electrical circuit andinterrupting the current path when a current fault condition isdetected.

In one currently preferred embodiment, the invention is a device forinterrupting a current path between the line side and the load side ofan electrical circuit. The device includes a power supply that iselectrically connected to the current path, either at the line side orthe load side. The device also includes a power controller that ispositioned within the current path. The power controller is adapted toclose the current path in the presence of a control supply and open thecurrent path in the absence of the control supply. The device furtherincludes a sensor system and a logic controller. The sensor systemreceives power from the power supply, monitors the current in thecurrent path and outputs a sensor signal indicative of a currentcondition within the current path. The logic controller also receivespower from the power supply, receives the sensor signal and removes thecontrol supply from the power controller when the sensor signal does notsatisfy an established criteria.

In a detailed aspect of the device, the control supply is provided bythe power supply and the power supply is adapted to, when the powercontroller is open and the sensor signal satisfies the establishedcriteria, output a control supply having a first voltage for a firstamount of time that sufficient to cause the power controller to closethe current path. After the first amount of time, the power supplyoutputs a control supply having a second voltage, less than the firstvoltage, sufficient to hold the power controller in the closed position.In other detailed facets of the device, the sensor system includeseither a single sensor associated with the three electrical lines of athree-phase AC system or the two lines of a DC system for providing acurrent balance measurement among the lines, or individual sensors, eachassociated with one of the electrical lines of the current path, forproviding individual current measurements for each electrical line, or acombination of both.

In another currently preferred embodiment, the invention is anintegrated current fault detection/circuit breaker that includes acircuit breaker adapted to be positioned within a current path having aline side and a load side and a power supply electrically connected tothe current path. The device also includes a sensor system that receivespower from the power supply, monitors the current in the current pathand outputs a sensor signal indicative of a current condition within thecurrent path. The device further includes a controller that receivespower from the power supply, receives the sensor signal and opens thecircuit breaker when the sensor signal does not satisfy an establishedcriteria.

In another aspect, the invention relates to a method of interrupting acurrent path between the line side and the load side of an electricalcircuit having a power supply electrically connected thereto. The methodincludes positioning a power controller in the current path. The powercontroller is adapted to be in a closed position when provided with acontrol supply and in an open position otherwise. Monitoring the currentin the current path and outputting a sensor signal indicative of acurrent condition within the current path using a sensor system andproviding the control supply to the power controller only when thesensor signal satisfies an established criteria.

In a detailed aspect, the method further includes monitoring the voltagelevel of the power provided to the sensor system by the power supply andignoring the sensor signal when the voltage level to the sensor systemis less than a predetermined value. By ignoring the sensor signal thecurrent fault detection aspect of the system is essentially inhibiteduntil such time the voltage supply is at or above the predeterminedvalue. In a related detailed aspect, the method further includesmonitoring an external on/off power switch and removing the controlsupply from the power controller when either of the following conditionsoccur: the sensor signal does not satisfy the established criteria, orthe external power switch is off. In yet another detailed facet, theestablished criteria is dependent on the operating current of theelectrical load connected to the load side which may have a firstoperating current for a first amount of time and a second operatingcurrent for a second amount of time. In this case, the method furtherincludes setting the established criteria to a first level based on thefirst operating current during the first amount of time and a secondlevel based on the second operating current during the second amount oftime.

In another aspect, the invention relates to a device for monitoring thecurrent path through an electrical circuit having a line side and a loadside and a power controller therebetween. The power controller closesthe current path in the presence of a control supply and opens thecurrent path in the absence of the control supply. The electricalcircuit is housed within a housing and the device includes a powersupply that is also housed within the housing and is electricallyconnected to the current path. The device also includes a sensor systemand a logic controller both of which are also housed within the housing.The sensor system receives power from the power supply, monitors thecurrent in the current path and outputs a sensor signal indicative of acurrent condition within the current path. The logic controller receivespower from the power supply, receives the sensor signal and removes thecontrol supply from the power controller when the sensor signal does notsatisfy an established criteria. In a detailed aspect, the devicefurther includes a flexible printed wiring board positioned around aportion of the power controller and circuitry comprising at least one ofthe power supply, sensor system and logic controller is mounted on theboard.

In another facet, the invention relates to a method of replacing anexisting power controller positioned between the load side and line sideof an electrical current path and housed within a housing having aspecific form factor. The method includes removing the existing powercontroller from the current path and providing a device adapted tomonitor a current condition through the current path and interrupt thecurrent path when the current condition does not satisfy an establishedcriteria. The device is housed within a housing having substantially thesame form factor as that of the removed power controller. The methodfurther includes installing the device within the current path at thelocation where the removed power controller was previously located.

In another facet, the invention relates to a device for closing acurrent path between the line side and the load side of an electricalcircuit. The device includes a power controller having an openedposition and a closed position. The power controller is located withinthe current path and switches from the open position to the closedposition in the presence of a first control supply and maintains theclosed position in the presence of a second control supply. The devicefurther includes a power supply which, when the power controller isopen, outputs the first control supply having a first voltage for afirst amount of time and after the first amount of time, outputs thesecond control supply having a second voltage, less than the firstvoltage and sufficient to hold the power controller in the closedposition.

In another aspect, the invention relates to a device for interrupting acurrent path between the line side and the load side of an electricalcircuit that is connected to an electrical load having an associatedfirst operating current for a first amount of time and a secondoperating current for a second amount of time. The device includes apower controller that is positioned within the current path. The powercontroller closes the current path in the presence of a control supplyand opens the current path in the absence of the control supply. Thedevice also includes a sensor system that monitors the current in thecurrent path and outputs a sensor signal indicative of a currentcondition within the current path. The device further includes a logiccontroller that receives the sensor signal and during the first amountof time compares the sensor signal to a first established criteriadefined by the first operating current and removes the control supplyfrom the power controller if the sensor signal does not satisfy thefirst established criteria. During the second amount of time, the logiccontroller compares the sensor signal to a second established criteriadefined by the second operating current and removes the control supplyfrom the power controller if the sensor signal does not satisfy thesecond established criteria.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description and the accompanyingdrawings which illustrate by way of example the features of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general block diagram of a system configured in accordancewith the invention including a power supply, a sensor system, a logiccontroller and a power controller;

FIG. 2 is block diagram of one configuration of the system including apower supply, a sensor system comprising a current imbalance sensor, alogic controller, an external DC pump input switch and a powercontroller comprising a DC relay;

FIGS. 3-1 through 3-3 form a schematic diagram of the power supply ofFIG. 2;

FIG. 4 is a schematic diagram of the current imbalance sensor of FIG. 2;

FIGS. 5 a-1 through 5 c-4 form a schematic diagram of the logiccontroller of FIG. 2;

FIG. 6 is a block diagram of another configuration of the systemincluding a power supply, a sensor system comprising a current imbalancesensor and three over-current sensors, a logic controller, an externalDC pump input switch and a power controller comprising a DC relay;

FIGS. 7-1 through 7-3 form a schematic diagram of the power supply ofFIG. 6;

FIGS. 8 a-1 through 8 c-4 form a schematic diagram of the logiccontroller of FIG. 6;

FIG. 9 is a block diagram of another configuration of the systemincluding a power supply, a sensor system comprising a current imbalancesensor, a logic controller, an external AC pump input switch and a powercontroller comprising a DC relay;

FIGS. 10 a and 10 b form a schematic diagram of the power supply of FIG.9;

FIG. 11 is a schematic diagram of the current imbalance sensor of FIG.9;

FIGS. 12-1 and 12-2 form a schematic diagram of the logic controller ofFIG. 9;

FIG. 13 is a block diagram of another configuration of the systemincluding a power supply, a sensor system comprising a current imbalancesensor and three over-current sensors, a logic controller, an externalAC pump input switch and a power controller comprising a DC relay;

FIG. 14 is a block diagram of another configuration of the systemincluding a power supply, a sensor system comprising a current imbalancesensor, a logic controller, an external AC pump input switch and a powercontroller comprising a AC relay;

FIG. 15 is a schematic diagram of the power supply of FIG. 14;

FIG. 16 is a schematic diagram of the current imbalance sensor of FIG.14;

FIGS. 17-1 and 17-2 form a schematic diagram of the logic controller ofFIG. 14;

FIG. 18 is a block diagram of another configuration of the systemincluding a power supply, a sensor system comprising a current imbalancesensor and three over-current sensors, a logic controller, an externalAC pump input switch and a power controller comprising a AC relay;

FIG. 19 a is a perspective view of a device configured in accordancewith the invention and adapted for use in the Boeing 737/747 Classic andAirbus aircraft;

FIG. 19 b through 19 d are top, front and bottom views respectively ofthe device of FIG. 19 a;

FIG. 19 e is a perspective view of the device of FIG. 19 a with acutaway showing components including the power controller, sensor systemand a flexible printed wiring board having system components mountedthereon;

FIG. 19 f is a planar view of the flexible printed wiring board of FIG.19 e;

FIG. 20 a is a perspective view of a device configured in accordancewith the invention and adapted for use in the Boeing 747-400, 757 and767 aircraft;

FIG. 20 b through 20 d are top, front and bottom views respectively ofthe device of FIG. 20 a;

FIG. 20 e is a perspective view of the device of FIG. 20 a with acutaway showing components including the power controller, sensor systemand printed wiring boards having system components mounted thereon;

FIG. 21 a is a perspective view of a device configured in accordancewith the invention and adapted for use in the DC-10, MD10 and MD11aircraft;

FIG. 21 b through 21 d are top, front and bottom views respectively ofthe device of FIG. 21 a;

FIG. 21 e is a perspective view of the device of FIG. 21 a with acutaway showing components including the power controller, sensor systemand a flexible printed wiring board having system components mountedthereon;

FIG. 21 f is a planar view of the flexible printed wiring board of FIG.21 e;

FIG. 22 is a block diagram of a current-fault detector/circuit breakerconfiguration of the system including a power supply, a sensor systemcomprising a current imbalance sensor, a solenoid drive and a powercontroller comprising a circuit breaker and a solenoid;

FIG. 23 is a schematic diagram of the system of FIG. 22;

FIG. 24 a is a perspective view of a current-fault detector/circuitbreaker configured in accordance with the invention;

FIG. 24 b through 24 d are top, front and bottom views respectively ofthe device of FIG. 24 a;

FIG. 24 e is a cutaway view of the device of FIG. 24 a showingcomponents including the circuit breaker and solenoid, sensor system anda printed wiring board having system components mounted thereon;

FIG. 25 is a DC standalone current-fault detector configuration of thesystem including a power supply, a sensor system comprising a currentimbalance sensor and an over-current sensor, and a power controllercomprising a DC relay;

FIG. 26 is a schematic diagram of the power supply of FIG. 25;

FIGS. 27 a-1 through 27 c-4 form a schematic diagram of the logiccontroller of FIG. 25;

FIG. 28 is a AC standalone current-fault detector configuration of thesystem including a power supply, a sensor system comprising a currentimbalance sensor and three over-current sensors and a power controllercomprising an AC relay;

FIG. 29 is a solid state standalone current-fault detector configurationof the system including a power supply, a sensor system comprising acurrent imbalance sensor and three over-current sensors, and a powercontroller comprising a solid state relay;

FIG. 30 a is a perspective view of a standalone current-fault detectordevice configured in accordance with the invention; and

FIGS. 30 b and 30 c are top and front views respectively of the deviceof FIG. 30 a;

FIG. 30 d is a perspective view of the device of FIG. 30 a with acutaway showing components including the power controller and a printedwiring board having system components mounted thereon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, which are provided for purposes ofillustration and not by way of limitation, and particularly to FIG. 1,there is shown a system 10 constructed in accordance with the presentinvention for interrupting a current path 20 between the line side 24and the load side 26 of an electrical circuit upon detection of acurrent fault-condition within the electrical circuit. A currentfault-condition may be the result of a current imbalance condition or anover-current condition within the electrical circuit.

In one of its most basic forms, the system 10 includes a power supply30, a sensor system 40, a logic controller 50 and a power controller 60.The power supply 30 provides power to the logic controller 50, thesensor system 40 and the power controller 60. The power controller 60may be an electro-mechanical relay, either AC coil or DC coil, or asolid state device. Coil type relays are powered by a control supply 32provided by the power supply. The return path 34 for the control supply32 passes through the logic controller 50. In other embodiments, thecontrol supply 32 is fed directly to the power controller 60 through anexternal switch instead of the power supply 30.

The sensor system 40 monitors the current passing through the currentpath 20 and outputs one or more sensor signals 42 indicative of currentconditions in the current path. The logic controller 50 receives the oneor more sensor signals 42 from the sensor system 40 and removes powerfrom the power controller 60 when at least one of the sensor signalsdoes not satisfy an established criteria. The established criteria isdescribed further below, but in general defines the boundaries ofacceptable current imbalance and over-current conditions within theelectrical circuit. When the sensor signal does not satisfy theestablished criteria, the logic controller 50 interrupts the return path34 of the control supply 32. This causes the power controller 60 tointerrupt the current path 20 thereby removing power to the load side 26of the circuit. The logic controller 50 includes circuitry and externalswitches related to fault indication and system testing and resetting.Upon interruption of the current path 20, the logic controller 50provides a fault indication which may be an illuminated LED or amechanical indicator (not shown). Mechanical indicators are beneficialin that they do not require electrical power, thus if the power to thesystem is shut off the fault is still indicated.

The system 10 of the invention is adaptable for use in various aircraftand various systems within the aircraft. For example the system may beincorporated into any one of the Boeing 737, 747, 757, 767, DC-10, MD11and Airbus fuel systems as a means of monitoring the power circuit beingused to supply power to the pumps within the aircraft fuel system. Thesystem 10 may also find application in other aircraft systems employingelectro-mechanical devices or solid state switches such as the brakingsystem which includes hydraulic pumps/motor and shut-off valves and theaircraft environment system which includes switch controlled lights,fans, ovens, etc.

The systems of the invention may be categorized into three generalconfigurations. The first general configuration is an integratedcurrent-fault protection/power controller. This configurationincorporates current-fault protection into a power control relay and isintended to replace existing aircraft system relays. The second generalconfiguration is an integrated current-fault protection/circuit breaker.This configuration incorporates current-fault protection into a circuitbreaker and is intended to replace existing aircraft system circuitbreakers. The third general configuration is a standalone current-faultprotection device. This configuration does not replace existing aircraftsystem components and instead is an additional device intended to beinstalled between existing aircraft circuit breakers and aircraft loads.Each of these three configurations is described below within the contextof aircraft fuel systems. Application of these systems, however, is notlimited to fuel systems.

Integrated Current-Fault Protection/Power Controller

Each of the pumps within a fuel system typically receives its power froma three phase AC power supply, via an electro-mechanical relay. Therelay itself is typically a DC coil relay, although an AC coil relay maybe used. DC coil relays are currently preferred due to their fasterresponse time, which is approximately 10-15 microseconds (usec) maximum.A typical AC coil relay has a response time of approximately 15-50 usec.However, AC coil relay designs that approach the response time of the DCcoil relay are currently being developed within the industry. Dependingon the type of relay, either an AC control supply or a DC control supplypowers the relay. These control supplies are at times referred to aspump inputs. The power input to the relay is typically switchablethrough a cockpit switch.

The system of the invention is intended to replace the relays used intypical aircraft fuel systems. To this end, the system may take anyoneof several configurations, depending on the type of relay being used inthe existing aircraft system and the type of switchable pump input. Forexample, the Boeing 757 and the DC-10 aircraft employ a DC relay coiland an AC pump input. The Boeing 737, 747 and 767 and Airbus aircraftemploy a DC relay coil and a DC pump input. For each of these existingaircraft configurations, the system includes a corresponding relay andthe circuitry necessary to power the relay. The system of the inventionis not limited to these existing aircraft system configurations and isfully adaptable to use in various contemplated systems. For example, thesystem may be configured to include an AC relay for use with either oneof an AC pump input or DC pump input. Following are descriptions ofvarious configurations of the system. For ease in describing theconfigurations, they are categorized based on their relay type and pumpinput type.

DC Relay with DC Pump Input

With reference to FIG. 2, there is shown a system 10 for use in anaircraft fuel system having a DC pump input 80 and an DC-coil relay 60,such as is present in the Boeing 737/747 Classic aircraft. The system 10includes a power supply 30 that taps off of each of the 115 VAC threephase lines at the input side 24 of the electrical circuit. The powersupply 30 provides power to the sensor system 40, the logic controller50 and the DC relay 60. The DC pump input 80 to the logic controller 50is provided by a cockpit pump switch 82 which receives power from anaircraft DC power source.

With reference to FIGS. 3-1 through 3-3, in one preferred embodiment,the power supply is a fly-back type switching power supply with twolinear power supplies U14 and U15. Power supply U14 provides 7 VDC tothe current sensor U7 (FIG. 4) in order to get the maximum gain out ofthe current sensor while power supply U15 provides 5 VDC to the logiccontroller circuitry (FIGS. 5 a-1 through 5 c-3). As explained laterbelow, the power supply also provides either 28 VDC or 16 VDC controlsupply to the power controller.

At the input side of the power supply, diodes CR12, CR13, CR14, CR15,CR16 and CR17 form a full-wave three-phase bridge. Capacitors C17 andC18 act as storage devices for the approximate 300V peak voltageproduced by the bridge. Resistors R42, R43 provide a filtering function,with resistor R42 and capacitor C17 functioning as one RC network andresistor R43 and capacitor C18 functioning as another RC network of atwo-pole filter. Resistors R44 and R45 provide EMI protection againstnoise going back out the input through any of diodes CR15, CR16 andCR17. Diodes VR2 and VR3 protect the control circuit U12 and transistorQ8 against voltage spikes that exceed their respective operatingcapabilities, which in one configuration are 450V and 800V.

Control circuit U12 senses the voltage across the primary winding oftransformer L2 through transistor Q8. Resistor R49 and capacitor C22filter noise going to the sense input of control circuit U12. If controlunit U12 senses the output voltage is low, it turns on and remains onuntil the current through transformer L2 reaches a predetermined amount.Resistor R50 allows the current going through transformer L2 to build upto the predetermined amount. Once the predetermined amount is reached,the device shuts off and energy is transferred to the secondary side oftransformer L2. At the secondary side, capacitor C26 filters out highfrequency noise while capacitor C25 stores the majority of the energy.Energy from the secondary side of transformer L2 is then provided tolinear power supplies U14 and U15.

Returning to FIG. 2, the sensor system 40 includes a single sensorsurrounding the three, three-phase electrical lines which form thecurrent path 20. The sensor 40 determines the current condition in thecurrent path 20 by providing an output sensor signal 42 indicative ofthe current balance among the electrical lines.

With reference to FIG. 4, in one embodiment, the sensor 40 is a Halleffect sensor such as an Amploc Pro 5 Hall effect linear current sensor.In alternate embodiments, the sensor 40 may be a current transformer ora giant magneto resistive (GMR) device. The sensor 40 has an output of233 mV/A when operated at 10V. Ground fault detection is accomplished bymonitoring the current of all three phases with the single currentsensor. The current sensor 40 algebraically sums the magnetic fluxgenerated by the three phase currents and produces an output signal 42that is proportional to the result. Since 3-phase AC fuel pumpstypically have an ungrounded neutral, the system is “closed”, requiringthe current going to the fuel pump to be equal and opposite the returncurrent. Therefore, when a ground fault condition does not exist, themagnetic flux measured at the current sensor is zero. When a groundfault condition occurs, current flows to ground (which does not returnthrough the sensor), breaking the closed loop system and resulting in amagnetic flux imbalance measured at the sensor. Since the flux imbalanceis proportional to the current, the output of the sensor provides themagnitude of the current loss. In a preferred embodiment, the output ofthe sensor is approximately one-half of the supply voltage, for nomeasured imbalance.

Returning to FIG. 2, the sensor signal outputs 42 from the sensorsystems 40 are received by the logic controller 50. The logic controller50 compares the sensor signals 42 against an established criteria andinterrupts the return path 34 of the power supply 32 if the criteria isnot satisfied. This removes the drive signal to the power controller 60and causes the DC relay to latch to a tripped condition and interruptthe current path 20 to the load side 26. The logic controller 50includes circuitry and external switches related to fault indication andsystem testing and resetting, these switches are not shown in FIG. 2 topreserve clarity of illustration.

With reference to FIGS. 5 a-1 through 5 a-2 the sensor output is inputto amplifier U1A, which adjusts the gain of the sensor output signal.Amplifier U1A also functions as a lowpass filter for the purposes ofprotecting against the threat of EMI or lightening. Resistors R6, R7 andR8 set the established criteria against which the sensor output signalis compared. Specifically, the resistors set the voltage referencelevels at pin 6 of U1B and pin 10 of U1C such that one voltagecorresponds to the upper threshold voltage while the other correspondsto the lower threshold voltage. These voltage levels in turn correspondto upper and lower current imbalance thresholds, which in one embodimentare +1.5 A RMS and −1.5 A RMS respectively.

If the voltage coming from amplifier U1A exceeds the upper thresholdvoltage of pin 6 or is below the lower threshold voltage of pin 10 theoutput of the corresponding amplifier U1B, U1C goes high which serves asa fault signal. A high output from either of these amplifiers brings thegate of transistor Q1 high which in turn drives transistor Q2 outputhigh transistor Q2 output passes through amplifier U2A. The output ofamplifier U2A is input to logic gate U10B (FIG. 5 b-3).

Logic gate U10B receives two additional inputs. One from a controlsupply circuit and one from a sensor power monitor circuit. In order forlogic gate U10B to operate, each of its inputs must be logic low.Regarding the control supply circuit (FIG. 5 b-1), the external DCcontrol supply passes through optical coupler U3, amplifier U2B andlogic gate U4A. In an alternate configuration, the control supplycircuit may be adapted to receive an external AC control supply bychanging the value of resistors R12 and R13 from 4.99 k to 49.9 k. Whenthe control supply is high, i.e., the pilot switch is on, the output ofU4A is low and logic gate U10B is allowed to operate provided the otherinputs to the gate are low.

With respect to the sensor power monitor circuit (FIG. 5 b-3),transistor Q7 monitors the 7 VDC power supply to the current sensor andinhibits the sensor output from effecting power controller operationwhen the voltage supply drops below a level where the sensor operatesproperly. When the supply voltage drops below the operating level,transistor Q7 turns on, which in turn inhibits logic gate U10B fromoperating. The system, in essence, ignores the sensor signal until thevoltage supply is at or above the predetermined value, at which timeoperation of logic gate U10B resumes.

The logic controller is designed so that upon power-up, the system isput into a fault condition during which it does not deliver power to theload. After the system is up and stabilized, the system switches intothe operate mode, provided that a fault doesn't exist. During reset, thesignal from logic gate U9A in the reset circuitry (FIG. 5 b-3) comesthrough U11A and goes to a latch (FIG. 5 b-4) which is made up of logicgates U9C and U9D, putting the latch in a fault condition for the first60 milliseconds (ms) to 100 ms depending on the values of resistor R31and capacitor C9 (FIG. 5 b-3) and the delay time of supervisory circuitU8 which is 140 to 560 ms. The longer delay controls the reset time. Thetwo methods are used due to limitation to each.

Latch U9C/U9D may also be put in a fault condition by a fault signalfrom logic gate U10B (FIG. 5 b-3). Such a fault would set the latchpermanently. A reset function is necessary to remove the fault from thelatch. Upon power-up reset, there is an inhibit for the historicallatch. If a fault comes through, latch U9C/U9D goes to the latchcondition and its output goes to U10C (FIG. 5 b-4). All inputs have tobe low for U10C to operate properly. If the output of logic gate U9C ishigh then a fault condition exists and the output of gate U10C is low.Such an output sets the gate of transistor Q3 (FIG. 5 c-1) low, whichinterrupts the control supply-to-ground path and deenergizes the relay,i.e., opens the relay. The output from U9C is also input to logic gatesU5B, U5C and U5D (FIG. 5 c-4), which in turn, set LED CR6 on when theoutput of logic gate U9C is high.

Logic gates U11C and U11D (FIG. 5 c-3), in conjunction with circuitrywithin the power supply, form a pop-up power supply. The pop-up powersupply provides a first control supply voltage to relay U6 (FIG. 5 c-1)when the relay is going from its open position to its closed position.Once the relay is closed for a certain amount of time, as defined by thevalues of capacitor C13 and resistor R40, the power supply provides asecond voltage, less than the first voltage, to relay U6 in order tokeep the relay closed. This reduces the heat dissipation in the powersupply and the relay coil.

When the output of gate UC10 (FIG. 5 b-4) is high, transistor Q3 turnson. This establishes the presence of the 28 VDC control supply to therelay by providing a return path for the control supply throughtransistor Q3. The presence of the control supply energizes relay U6 andcauses it to close. At the onset of the command to close the relay, theoutput of logic gate U11D goes high and is feed back into logic gateU11C. When the output of gate U11D goes high, transistor Q9 (FIG. 3-3)in the power supply turns on and it pulls current through resistor R54,which shunts resistor R52. This causes the voltage at the top ofresistor R53 to go to 28 VDC. When resistor R52 is not shunted, thevoltage across resistors R52 and R53 is about 16 VDC. Thus the pop-uppower supply provides 28 VDC to pull the relay in, i.e., to switch itfrom an opened to a closed position, and 16 VDC to maintain relay U6 inthe closed position. At a time determined by resistor R40 and capacitorC13 (FIG. 5 c-3), after the onset of the command to close the relay, theoutput of gate U11D goes low, which turns off Q9 returning the voltageto 16 VDC.

For devices that do not have over-current protection, the presentapproach would be to maintain the first voltage until the powercontroller closed and revert to the first voltage if the powercontroller opened. Otherwise it would maintain the second voltage aslong as the power controller was closed. With over-current this would beaugmented by a first voltage for a minimum first amount of time.

During a fault condition, the output of logic gate UC9 (FIG. 5 b-4) ishigh. Following this output through logic gates U5B and U5C (FIG. 5c-4), the output of U5C is set high. This causes transistor Q6 to turnon and LED CR6 to light. The high output of gate U5C is input to logicgate U10A (FIG. 5 c-2) which functions as an inverter and outputs alogic low to pin II of gate U4D. Pin 12 of gate U4D is also low. Thusthe output of gate U4D drives transistor Q5, which in turn driveslatching relay U7. Pin 3 of the latching relay switches to contact pin 2and thereby provides 5 VDC to pin 12 of gate U4D which causes the outputof U4D to go low. When the output of gate U4D goes low, transistor Q5turns off conserving logic power.

Once in a fault condition, reset switch S2 (FIG. 5 b-1) may be used toreset the logic controller. When switch S2 is closed, the output ofamplifier U2C goes high and the output of U4B (FIG. 5 c-1) goes low. Atgate U4C (FIG. 5 c-2), both inputs are low so the output is high. A highoutput from gate U4C turns transistor Q4 on, which in turn driveslatching relay U7 to switch contact from pins 3 and 2 to pins 3 and 1.This causes the output of gate U4C to go low and transistor Q4 to turnoff.

The logic controller includes various maintenance circuitry includingthe previously described reset switch S2 and fault indication LED CR6.Also includes is press-to-test circuitry (FIG. 5 c-1) which includescoil L1, resistors R25 and R26, and switch S1. The coil is wrappedaround the current sensor (not shown) a sufficient number of times suchthat when switch S1 is closed the sensor outputs a signal indicative ofa current imbalance. In one configuration, the coil is wrapped aroundthe sensor 25 times.

The system thus described monitors the current path 20 for a currentimbalance and provides GFI protection to the load. However, currentfault-conditions undetectable by a single sensor may be present in theelectrical circuit. For example, if a short occurs across any two of thethree electrical lines downstream from the single sensor, the summationof the current passing through the sensor may still be zero. Thus theshort will go undetected. In accordance with the invention, theforegoing is guarded against by including over-current sensors as partof the sensor system.

With reference to FIG. 6, there is shown such a system 10 for use in anaircraft fuel system having a DC pump input 80 and an DC-coil relay 60,such as is present in the Boeing 747-400 or Boeing 767 aircraft. Thesystem 10 includes a power supply 30 that taps off of each of the 115VAC three phase lines at the input side 24 of the electrical circuit.The power supply 30 provides power to each sensor within the sensorsystem 40, the logic controller 50 and the DC relay 60.

The power supply is a fly-back type switching power supply, theconfiguration and operation of which is similar to that previouslydescribed with reference to FIGS. 3-1 through 3-3. The power supplyincludes two linear power supplies U14, U15. Power supply U14 provides 7VDC to each of the current sensors in the sensor system while powersupply U15 provides 5 VDC to the logic controller circuitry. The powersupply also functions as a pop-up power supply which provides either 28VDC or 16 VDC control supply to the DC relay. The DC pump input 80 tothe logic controller 50 is provided by a cockpit pump switch 82 whichreceives power from an aircraft DC power source. A detailed schematic ofan exemplary power supply is provided in FIGS. 7-1 through 7-3.

The sensor system 40 includes a single imbalance sensor 44 surroundingthe three, three-phase electrical lines which form the current path 20.The sensor system 40 also includes three over-current sensors 46. Eachof the over-current sensors 46 surrounds one of the three electricallines forming the current path 20. Both the imbalance sensor 44 and theover-current sensors 46 may be Hall effect sensors such as previouslydescribed with reference to FIG. 4. The imbalance sensor 44algebraically sums the magnetic flux generated by the three phasecurrents through the three phase electrical lines and produces an outputsignal 42 that is proportional to the result. Each of the over-currentsensors 46 outputs a signal 48 indicative of the amount of currentpassing through its associated electrical line. In alternateconfiguration, the sensors 44, 46 may also be a current transformer or aGMR sensor.

With continued reference to FIG. 6, signals 42, 48 from the imbalancesensor 44 and the over-current sensors 46 are provided to the logiccontroller 50 where they are compared against respective preestablishedcriteria. With respect to the imbalance sensor 44 the criteria issimilar to that previously described with reference to FIG. 2, namely,−1.5 A RMS to +1.5 A RMS. With respect to the over-current sensors 46,the criteria is a function of the electrical load connected to the loadside 26 of the circuit. In one embodiment, the threshold is 1.25× theoperating current of the load. In a preferred embodiment, the logic ishardware implemented. Alternatively, the logic may be provided byprogrammable firmware. In either case, the logic is such that if any ofthe sensor signals 42, 48 does not satisfy the preestablished criteria,the return path 34 of the control supply 32 is interrupted. This removesthe drive signal to the power controller 60 and causes the DC relay tolatch to a tripped condition and interrupt the current path 20 to theload side 26.

In a preferred embodiment, the system is configured to provide atwo-tier threshold criteria for detecting over-current fault conditions.One criteria is applicable during normal operation of the load, whilethe other is applicable during power-up operation of the load. Theover-current threshold for each is different. In normal operation the DCrelay is closed and 115 VAC is being provided to the pump motor whichhas an associated steady-state operating current. During normaloperation, the system detects the over-current condition using athreshold based on the steady-state operating current. For example, ifthe steady-state operating current of the motor is 5 A, the establishedthreshold is 1.25×5 A RMS.

In start-up operation the load is off and then power is applied byclosing the DC relay. During start-up operation, the system detects theover-current condition using a threshold established based on thestart-up current of the load. For example, if the start-up current is 20A, the established threshold is 1.25×20 A RMS. The system uses thisstart-up threshold for a certain period of time, i.e., the start-upperiod, before switching to the normal threshold. The duration of thestart-up period is based on the time it takes for the load to power upand stabilize and may range from, for example, approximately 80 ms.Thus, if the system detects a current over the start-up threshold duringthe start-up time period, the relay is opened and power is removed fromthe load. A benefit of the two-tier threshold system thus described isthat it prevents nuisance trips during start-up of the load and allowsclose monitoring during normal operation.

With reference to FIGS. 8 a-1 through 8 c-4, the sensor controlcircuitry of the logic controller is similar to that previouslydescribed with reference to FIGS. 5 a-1 through 5 c-4. The imbalancesensor output is input to amplifier U1A (FIG. 8 a-1), which adjusts thegain of the sensor output signal. Resistors R6, R7 and R8 (FIG. 8 a-2)set the established criteria against which the sensor output signal iscompared. Specifically, the resistors set the voltage reference levelsat pin 6 of U1B and pin 10 of U1C such that one voltage corresponds tothe upper threshold voltage while the other corresponds to the lowerthreshold voltage. These voltage levels in turn correspond to upper andlower current imbalance thresholds, which in one embodiment are +1.5 ARMS and −1.5 A RMS respectively.

If the voltage coming from amplifier U1A exceeds the upper thresholdvoltage of pin 6 or is below the lower threshold voltage of pin 10 theoutput of the corresponding amplifier U1B, U1C goes high. A high outputfrom either of these amplifiers brings the gate of transistor Q2 (FIG. 8a-4) low which in turn drives transistor Q4 output high. Transistor Q3output passes through amplifier U4D. The output of amplifier U4D isinput to logic gate U8.

Each of the over-current sensor outputs are input to an amplifier U2A,U3A, U4A (FIGS. 8 a-1 and 8 a-3), which adjusts the gain of the sensoroutput signal. Each amplifier output is input to a pair of comparatorsU2B/U2C, U3B/U3C, U4B/U4C (FIGS. 8 a-2 and 8 a-4) which function asover-current detectors for each of line A, B and C of the three phasecurrent path.

As previously mentioned, the logic controller is configured to provide astart-up threshold and a normal threshold for the over-currentdetectors. These thresholds are set by resistors R76 and R77. Duringnormal operation, component U5A (FIG. 8 a-3) is on, shorting-outresistor R77 (FIG. 8 a-4). The voltage developed across resistor R76 issmall, thus setting the over-current threshold to the normal thresholdvalue. In one configuration, R76 is 24.3 k and the normal thresholdvalue is 15 A. If any one of the three over-current amplifiers U2A, U3A,U4A outputs a signal having a voltage greater than the voltagecorresponding to the normal threshold value, the output of theover-current detector associated with the amplifier goes high. A highoutput from any of these over-current detectors brings the gate oftransistor Q2 low which in turn drives transistor Q3 output high.Transistor Q2 output passes through amplifier U4D. The output ofamplifier U4D is input to logic gate U8.

During start-up operation, component U5A is off and the voltagedeveloped across resistors R76 and R77 is larger, thus setting theover-current threshold to the start-up value. In one configuration,resistor R76 is 24.3 k and resistor R77 is 127 k and the start-upthreshold value is 60 A. The on/off operation of component U5A is linkedto the operation of the previously described pop-up power supply throughtransistors Q1 and Q9 (FIGS. 8 a-3 and 7-3). The duration of time forstart-up operation, i.e., the time during which component U5A is off, isdetermined by the capacitor C53 (FIG. 8 c-3) and resistor R111. Forexample, with a capacitor C53 of one microfarad and a resistor R111 of100K, the start-up time is approximately 70 ms.

The rest of the logic controller circuitry, as shown in FIGS. 8 b-1through 8 c-4 is similar to that previously described with reference toFIGS. 5 b-1 through 5 c-4. Though not shown in the schematic, the logiccontroller may include various maintenance circuitry including the areset switch and a press-to-test switch and circuitry similar to thatpreviously described with reference to FIGS. 5 b-1 and 5 c-1.

DC Relay with AC Pump Input

With reference to FIG. 9, there is shown a system for use in an aircraftfuel system having an AC pump input 80 and an DC-coil relay 60, such asis present in the Boeing 757 aircraft. The system includes a powersupply 30 that taps off of each of the 115 VAC three phase lines at theinput side 24 of the electrical circuit. The power supply 30 providespower to the sensor system 40, the logic controller 50 and the DC relay60. The AC pump input 80 to the logic controller 50 is provided by acockpit pump switch 82 which taps off of one of the three phase lines.

With reference to FIGS. 10 a and 10 b the power supply includes a 10 VSupply (FIG. 10 a) and a 20 V supply (FIG. 10 b). The power supplyincludes diodes CR1, CR2, CR3, CR4, CR5, and CR6 that form a full-wavethree-phase bridge. Capacitor C1 acts as the storage device for the 281Vpeak voltage produced by the bridge. The regulators are a buck-typeconfiguration with the abnormal architecture of having the inductor inthe lower side. This is acceptable because the circuit does not have tobe referenced to earth ground. In fact, the on-board electrical groundis approximately 270V and 260V above earth ground for the 10V and 20Vsupplies respectively.

Preferably, the switcher operates in a non-conventional mode. If it issensed that an output voltage is low, the corresponding controller turnson and remains on until the current through inductor L1 or L1A reaches apre-determined amount. Otherwise, the cycle is skipped. Energy is storedin inductor L1 or L1A and transferred to output capacitor C3 or C3Athrough diode CR7 or CR7A. Proper regulation is determined by Zener VR1or VR1A and opto-coupler U2 or U2A. Capacitor C2 or C2A serves to storea small amount of energy that each respective regulator uses to operateits internal circuitry.

Returning to FIG. 9, the sensor system 40 includes a single sensorsurrounding the three, three-phase electrical lines which form thecurrent path 20. The sensor 40 determines the current condition in thecurrent path 20 by providing an output sensor signal 42 indicative ofthe current balance among the electrical lines.

With reference to FIG. 11, in one embodiment, the sensor 40 is a Halleffect sensor such as an Amploc Pro 5 Hall effect linear current sensor.In alternate embodiments, the sensor 40 may be a current transformer ora giant magneto resistive (GMR) device. The sensor 40 has an output of233 mV/A when operated at 10V. Ground fault detection is accomplished bymonitoring the current of all three phases with the single currentsensor. The current sensor 40 algebraically sums the magnetic fluxgenerated by the three phase currents and produces an output signal 42that is proportional to the result. Since 3-phase AC fuel pumpstypically have an ungrounded neutral, the system is “closed”, requiringthe current going to the fuel pump to be equal and opposite the returncurrent. Therefore, when a ground fault condition does not exist, themagnetic flux measured at the current sensor is zero. When a groundfault condition occurs, current flows to ground (which does not returnthrough the sensor), breaking the closed loop system and resulting in amagnetic flux imbalance measured at the sensor. Since the flux imbalanceis proportional to the current, the output of the sensor provides themagnitude of the current loss. In a preferred embodiment, the output ofthe sensor is approximately one-half of the supply voltage, for nomeasured imbalance.

Returning to FIG. 9, the sensor signal output 42 from the sensor system40 is received by the logic controller 50. The logic controller 50compares the sensor signals 42 against an established criteria andinterrupts the return path 34 of the power supply 32 to the powercontroller if the criteria is not satisfied. This removes the drivesignal to the power controller and causes the DC relay to latch to atripped condition and interrupt the current path 20 to the load side 26.

With reference to FIGS. 12-1 and 12-2, in a preferred embodiment, theoutput of the sensor is approximately one-half of the supply voltage,for no measured imbalance. Amplifier U3A amplifies the signal by afactor of 10. The gain is set by the ratio of resistors R5 and R3. The 3db point is where the reactance of capacitor C4 is equal to theresistance of R5. This occurs at 3386 Hz. Resistors R1, R2, and R4 biasthe amplifier and have been selected so that a maximum value of 1 meg,for resistor R4, is required to adjust the amplifier output to midsupply with the sensor at its specified worse case high output.Calibration for the worse case low output of the sensor is easilyachieved.

Amplifiers U3B and U3C, and resistors R6, R7, and R8 are set to detect acurrent imbalance outside upper and lower current thresholds, which inone embodiment are +1.5 A RMS and −1.5 A RMS respectively. A high outputfrom amplifier U3B or U3C indicates an imbalance is present in excess ofthe current thresholds. Gate U4A “OR's” the outputs from amplifiers U3Band U3C. A logic 0 at its output indicates one or the other failurecondition is present. Simultaneous imbalance inputs can be handled butare physically not possible since a positive imbalance cannot exist atthe same time as a negative imbalance.

If a fault condition exists, it passes through gate U5A presenting alogic 1 to the latch comprised of gates U4B and U4C. A logic 1, at pin5, forces the output pin 4 low, turning transistor Q1 off, whichinterrupts the return path of the power supply to the DC relay therebyeffectively removing the drive signal to the DC relay causing it to openand interrupt the current path 20 to the load side 26. Pin 9, the otherinput to the latch, is normally at logic 0. This causes pin 10 to gohigh, setting the latch by presenting a logic 1 to pin 6.

In a preferred embodiment, the power-up sequence initializes the powercontrol section to the non-operate mode. This is accomplished bypresenting a logic 0 to pin 2 of gate U5A to mimic a current imbalancecondition. The power-up reset pulse created by gate U5B, resistor R13,capacitor C5 and diode CR8 is typically 7 usec. The reset is determinedby the time it takes to charge capacitor C5 through resistor R13 to thethreshold set by gate U5B. Diode CR8 provides a quick reset.

With reference to FIG. 13, the system of FIG. 9 may be modified toinclude a sensor system 40 having over-current sensors 46 for monitoringthe phase-to-phase current. The configuration of such a system issimilar to that previously described with reference to FIG. 6.

AC Relay with AC Pump Input

With reference to FIG. 14, there is shown a system for use in anaircraft fuel system having an AC pump input and an AC-coil relay 60.The system includes a power supply 30 that taps off of each of the 115VAC three phase lines at the input side 24 of the electrical circuit.The power supply 30 provides power to the sensor system 40 and the logiccontroller 50. The AC pump input 80 for the relay 60 is provided by acockpit pump switch 82 which taps off of one of the three phase lines.

With reference to FIG. 15, in one embodiment of the power supply 30,diodes CR1, CR2, CR3, CR4, CR5, and CR6 form a full-wave three-phasebridge. Capacitor C1 acts as the storage device for the 281V peakvoltage produced by the bridge. The regulator is preferably a buck-typeconfiguration with the abnormal architecture of having the inductor inthe lower side. This is acceptable because the circuit does not have tobe referenced to earth ground. In fact, the on-board electrical groundis approximately 270 V above earth ground.

Preferably, the switcher operates in a non-conventional mode. If itsenses that output voltage is low, it turns on and remains on until thecurrent through inductor L1 reaches a pre-determined amount. Otherwise,the cycle is skipped. Energy is stored in inductor L1 and transferred tooutput capacitor C3 through diode CR7. Proper regulation is determinedby Zener VR1 and opto-coupler U2. Capacitor C2 serves to store a smallamount of energy that the regulator uses to operate its internalcircuitry.

Returning to FIG. 14, the sensor system 40 includes a single sensorsurrounding the three, three-phase electrical lines which form thecurrent path 20. The sensor 40 determines the current condition in thecurrent path 20 by providing an output sensor signal 42 indicative ofthe current balance among the electrical lines.

With reference to FIG. 16, in one embodiment, the sensor 40 is a Halleffect sensor such as an Amploc Pro 5 Hall effect linear current sensor.In alternate embodiments, the sensor 40 may be a current transformer ora giant magneto resistive (GMR) device. The sensor 40 has an output of233 mV/A when operated at 10V. Ground fault detection is accomplished bymonitoring the current of all three phases with the single currentsensor 40. The current sensor 40 algebraically sums the magnetic fluxgenerated by the three phase currents and produces an output signal 42that is proportional to the result. Since 3-phase AC fuel pumpstypically have an ungrounded neutral, the system is “closed”, requiringthe current going to the fuel pump to be equal and opposite the returncurrent. Therefore, when a ground fault condition does not exist, themagnetic flux measured at the sensor 40 is zero. When a ground faultcondition occurs, current flows to ground (which does not return throughthe sensor), breaking the closed loop system and resulting in a magneticflux imbalance measured at the sensor 40. Since the flux imbalance isproportional to the current, the output of the sensor 40 provides themagnitude of the current loss. In a preferred embodiment, the output ofthe sensor 40 is approximately one-half of the supply voltage, for nomeasured imbalance.

Returning to FIG. 14, the sensor signal output 42 from the sensor system40 is received by the logic controller 50. The logic controller 50compares the sensor signal 40 against an established criteria andinterrupts the return path 34 of the power supply 32 to the powercontroller if the criteria is not satisfied.

Referring to FIGS. 17-1, 17-2, amplifier U3A of the logic controller 50receives the sensor signal 42 and amplifies the signal by a factor often. The gain is set by the ratio of resistors R5 and R3. The 3 db pointis where the reactance of capacitor C4 is equal to the resistance of R5.This occurs at 3386 Hz. Resistors R1, R2, and R4 bias the amplifier andhave been selected so that a maximum value of 1 meg, for resistor R4, isrequired to adjust the amplifier output to mid supply with the sensor atits specified worse case high output. Calibration for the worse case lowoutput of the sensor is easily achieved.

Amplifiers U3B and U3C, and resistors R6, R7, and R8 are set to detect acurrent imbalance outside upper and lower current thresholds, which inone embodiment are +1.5 A RMS and −1.5 A RMS respectively. A high outputfrom amplifier U3B or U3C indicates an imbalance is present in excess ofthe 1.5 A RMS threshold. IC U4A “OR's” the outputs from amplifiers U3Band U3C. A logic 0 at its output indicates one or the other failurecondition is present. Simultaneous imbalance inputs can be handled butare physically not possible since a positive imbalance cannot exist atthe same time as a negative imbalance.

If a fault condition exists, it passes through IC U5A presenting a logic1 to the latch comprised of ICs U4B and U4C. A logic 1, at pin 5, forcesthe output pin 4 low, turning transistor Q1 off, which interrupts thereturn path 34 of the control supply 32 to the power controller 60,thereby removing the drive signal to the power controller and causingthe AC relay to latch to a tripped, i.e., open, condition and interruptthe current path 20 to the load side 26. Pin 9, the other input to thelatch, is normally at logic 0. This causes pin 10 to go high, settingthe latch by presenting a logic 1 to pin 6.

In a preferred embodiment, the power-up sequence initializes the powercontrol section to the non-operate mode. This is accomplished bypresenting a logic 0 to pin 2 of IC U5A to mimic a current imbalancecondition. The power-up reset pulse created by IC U5B, resistor R13,capacitor C5 and diode CR8 is typically 7 usec. The reset is determinedby the time it takes to charge capacitor C5 through resistor R13 to thethreshold set by IC U5B. Diode CR8 provides a quick reset.

With reference to FIG. 18, the system of FIG. 9 may be modified toinclude a sensor system 40 having over-current sensors 46 for monitoringthe phase-to-phase current. The configuration of such a system issimilar to that previously described with reference to FIG. 6.

Packaging

Most aircraft presently in service utilize circuit breakers with thelimitations previously described. While the electronic andelectromechanical aspects of the present invention impart additionalprotection to the protection provided by such circuit breakers, it wouldbe desirable to be able to package the invention in a form which wouldallow ease of retrofit to existing aircraft, newly constructed and newaircraft designs, thus bringing the benefits of the invention to a widerrange of applications. Accordingly, in a further presently preferredaspect of the invention, the electronic and electromechanical elementsof the system are housed within a housing which has a similar formfactor to existing power controllers. The system connects with thecircuit to be monitored and controlled through the existing powercontroller electrical connector and draws power from the circuit to bemaintained. While there are numerous form factors which can impartadditional protection to the protection provided by such circuitbreakers, variants of the integrated current fault interrupter are basedon: a relay and a current fault interrupter circuit, or a solid stateswitching device and a current fault interrupter circuit. The integratedcurrent fault interrupter is customized for specific aircraftinstallations. The fit and form are tailored to accommodate specificrelay installations in the aircraft.

With reference to FIGS. 19 a-19 d, some of the above-describedintegrated current-fault protection/power controllers can be configuredto comply with the form factor of existing power controller housings 90used in the Boeing 737 Classic, 747 Classic and Airbus aircraft. Suchhousings 90 typically include a connector portion 92, a mounting flange94 and a cover 96. The approximate dimensions of the housing 90 are asfollows: approximately 2.65 inches (about 6.73 cm.) from top 98 tobottom 100, approximately 1.50 inches (about 3.81 cm.) wide along itssides 102 and approximately 2.0 inches (about 5.08 cm.) from the frontside 104 of the mounting flange 94 to the rear side 106 of the mountingflange.

The connector portion 92 includes electrical connector means such as aterminal block or connector plate 108, typically with eight screw-typeelectrical connectors A1, A2, X1, B1, B2, C1, C2, and X2, although otherconventional types of wire connectors may also be suitable. Referring toFIGS. 2 and 6, the connectors A1 and A2 accommodate a first line A andload A, the connectors B1 and B2 accommodate a second line B and load B,and the connectors C1 and C2 accommodate a third line C and load C.

Referring to FIGS. 19 c and 19 f, the circuitry forming the power supply30, the sensor system 40, the logic controller 50 and the powercontroller 60 is mounted to a flex circuit board 110. The flex circuitboard 110 includes portions of rigid circuit board 112 joined togetherby flexible portions 114. The circuit board 110 is folded into arectangular form that fits within the housing cover 96 as shown in FIG.19 c. The use of the flex circuit board allows for the system circuitryto fit within the housing having the same form factor as the part whichit is replacing. A fault indicator 116 and Reset and Test buttons arelocated on the top exterior of the cover.

With reference to FIGS. 20 a-20 f, some of the above-describedintegrated current-fault protection/power controllers can be configuredto comply with the form factor of existing power controller housings 120used in the Boeing 747-400, 757 and 767 aircraft. Such housings 120typically include a connector portion 122, a mounting flange 124 and acover 126. The approximate dimensions of the housing 120 are as follows:approximately 3.28 inches (about 8.33 cm.) from top 128 to bottom 130,approximately 1.53 inches (about 3.89 cm.) wide along its short sides132, and approximately 2.51 inches (about 6.38 cm.) along its long sides134.

The connector portion 122 includes electrical connector means such as aterminal block or connector plate 136, typically with eight screw-typeelectrical connectors, A1, A2, X1, B1, B2, C1, C2, and X2, althoughother conventional types of wire connectors may also be suitable.Referring to FIGS. 9 and 13, the connectors A1 and A2 accommodate afirst line A and load A, the connectors B1 and B2 accommodate a secondline B and load B, and the connectors C1 and C2 accommodate a third lineC and load C.

Referring to FIG. 20 e, the circuitry forming the power supply 30 andthe logic controller 50 is mounted on two circuit boards 138, 140 whichare positioned above the sensor system 40 and the power controller 60. Afault indicator 142 and Reset and Test buttons are located on the topexterior of the cover.

With reference to FIGS. 21 a-21 d, some of the above-describedintegrated current-fault protection/power controllers can be configuredto comply with the form factor of existing power controller housings 150used in the DC-10 aircraft. Such housings 150 typically include aconnector portion 152, a mounting flange 154 and a cover 156. Theapproximate dimensions of the housing 150 are as follows: approximately3.25 inches (about 8.26 cm.) from top 158 to bottom 160 andapproximately 2.5 inches (about 6.35 cm.) wide along its sides 162.

The connector portion 152 includes electrical connector means such as aterminal block or connector plate 164, typically with eight screw-typeelectrical connectors, A1, A2, X1, B1, B2, C1, C2, and X2, althoughother conventional types of wire connectors may also be suitable.Referring to FIGS. 9 and 13, the connectors A1 and A2 accommodate afirst line A and load A, the connectors B1 and B2 accommodate a secondline B and load B, and the connectors C1 and C2 accommodate a third lineC and load C.

Referring to FIGS. 21 e and 21 f, the circuitry forming the power supply30, the sensor system 40, the logic controller 50 and the powercontroller 60 is mounted to a flex circuit board 166. The flex circuitboard 166 includes portions of rigid circuit board 168 joined togetherby flexible portions 170. The circuit board 166 is folded into arectangular form that fits within the housing cover 156. The maintenancesystem 70 circuitry is located near the top of the cover and interfaceswith a fault indicator 172 and Reset and Test buttons located on the topexterior of the cover.

Integrated Current-Fault Protection/Circuit Breaker

In another embodiment, the invention provides an integratedcurrent-fault protection circuit breaker, referred to herein as a faultprotection breaker (FPB). The FBD incorporates the current faultprotection aspects of the previously described integrated current-faultprotection/power controllers into a circuit breaker that is intended toreplace existing aircraft system circuit breakers. The FPB consists ofsensing and control electronics and electromechanical componentsintegrated with a circuit breaker in a single package to provide theaddition of ground fault and/or over-current detection while maintainingthe existing capabilities and functionality of the 3-phase 115 VACcircuit breaker. An extension push-pull button, herein referred to asthe FPB push-pull button is attached to the internal circuit breakerpush-pull button, and when this extension is electromechanicallytripped, it pulls open the circuit breaker push-pull button. In apreferred embodiment, the electromechanical trip is done with a solenoidand balance spring to provide the necessary forces to pull open thecircuit breaker. Manual operation of the FPB push-pull button can alsobe performed to open and close the circuit breaker contacts to remove orapply power to the fuel pump.

With reference to FIG. 22, in one configuration, the FPB includes apower supply 100, a current sensor 110, a controller or solenoid drive120, a solenoid 130, a mechanical circuit breaker 140 with associatedpush-pull button (not shown) and a test switch 150. The FPB push-pullbutton is electromechanically tripped to latch the ground faultcondition, removing the 3-phase power to the fuel pump and providing FPBindication. The FPB is a completely self-contained unit, obtaining itselectronics power from the load side 3-phase power and uses the sameaircraft wire interfaces as the circuit breaker.

With reference to FIG. 23, the FPB detects net flux generated by thethree phase currents using a single current sensor L2. The flux signalis input to the solenoid drive Q1 where it is conditioned and filteredto eliminate nuisance trips and the net fluxes of either polarity aredetected. Upon the occurrence of a ground fault the solenoid drive Q1causes the solenoid L1 to trigger an electromechanical activation of thepush-pull button, removing the 3-phase power. The FPB push-pull buttonremains in the latched faulted condition until the push-pull button ismanually reset. When the FPB is in the faulted state, cycling the pumpcontrol switch 160 (115 VAC or 28 VDC) has no effect on control of thefuel pump.

In one configuration, the FPB detects a ground fault condition when acurrent imbalance exists that is outside of an established range, whichin one configuration is −1.5 A RMS to +1.5 A RMS. This range allowssufficient margin to prevent fuel pump motor leakage current that is afunction of motor winding insulations and contaminated fuel, fromcausing nuisance trips.

As in the case of determining fault detection thresholds, nuisance tripsare also a concern when choosing fault detection speed. The FPB responseto a ground fault can be broken into detection and reaction times. Thereaction time is dependent on the electromechanical actuation of thepush-pull button, while the detection time is dependent on minimizingnuisance trips. The FPB includes a filtering system to attenuate highfrequency effects most commonly caused by noise, EMI, lightning andHIRF. Typically these high frequency signals cause erroneousindications. The bandwidth of the system and the range of operation havebeen restricted to minimize nuisance trips while retaining the widestpossible frequency spectrum of detection. A sensor 110 with the properrange of sensitivity is selected to allow signals in the range ofinterest to be measured.

FPB operation is lost in the event of a power interruption of the3-phase power to the line side of the FPB. However, power interruptionsof any duration, do not result in a latched FPB fault condition or resetof a latched FPB fault condition. When the interrupt terminates, the FPBreturns to the mode it was in prior to the interruption.

A ground fault condition trips the FPB push-pull button located on thetop of the FPB, extending the button to a height to provide visualindication of the FPB state. The FPB in a tripped condition can only bereset by manually resetting the push-pull button 142 on the FPB.

In a preferred embodiment, an end-to-end test verifies the FPB isoperating correctly. This is accomplished with a press-to-test switch150 located on the top of the FPB housing. Activation of this switchsimulates a ground fault condition by passing a current through thesensor that is just above the maximum threshold. This manual testresults in a latched FPB, removes power from the load side contacts andextends the push-pull button to provide visual indication. The push-pullbutton must be reset (pushed in) to clear the FPB fault condition.

Packaging

With reference to FIGS. 24 a-24 f, the above-described integratedcurrent-fault protection/circuit breaker can be configured to complywith the form factor of existing 15A and 20A circuit breaker housings180 used in the Boeing 700 series aircraft and the like. Such housings180 typically include a connector portion 182, a cover 184 and apush/pull button 142. The approximate dimensions of a 15 A circuitbreaker housing 180 are as follows: approximately 4.00 inches (about10.16 cm.) from top 188 to bottom 190, approximately 2.00 inches (about5.08 cm.) along its long sides 192 and approximately 1.38 inches (about3.51 cm.) along its short sides 194.

The connector portion 182 includes electrical connector means such as aterminal block or connector plate with six terminals, A1, A2, B1, B2, C1and C2. Referring to FIG. 23, the connectors A1 and A2 accommodate afirst line A and load A, the connectors B1 and B2 accommodate a secondline B and load B, and the connectors C1 and C2 accommodate a third lineC and load C. The sensor system 110 is positioned above the connectorportion 182. The input lines A, B and C pass through the sensor system110 and are input to a circuit breaker which functions as the powercontroller 140. The output lines A, B and C return to the connectorportion.

The circuitry forming the power supply 100, and the solenoid drive 120is mounted to a circuit board 196. The power supply 100 and solenoiddrive 120 interface with the solenoid 130.

Standalone Current-Fault Protection Device

In accordance with another embodiment of the invention, the system isconfigured as a standalone device that can be installed anywhere in acircuit between a circuit breaker and the load, as long as the power andreturn lines associated with the load are accessible to the device. Aswith the previously described system configurations, the standalonedevices derive their operational power from the line side of the powerline being monitored. Accordingly, no additional power sources arerequired by the device.

DC Configuration

With reference to FIG. 25, the system is adapted to monitor the currentpath between a DC power source and a load powered by the power source.The system includes a power supply 30 that taps off of the DC line andthe return line at the input side 24 of the electrical circuit. Thepower supply 30 provides power to the sensor system 40, the logiccontroller 50 and the power controller 60. The sensor system 40 includesone imbalance sensor 44 and one over-current sensor 46.

With reference to FIG. 26, in one preferred embodiment, the power supplyhas two linear power supplies U1 and U2. Power supply U1 provides 7 VDCto the current sensors 44, 46 (FIG. 25) in order to get the maximum gainout of the current sensor while power supply U2 provides 5 VDC to thelogic controller circuitry.

At the input side of the power supply, 28 VDC are received from theaircraft. Diode CR1 acts as a blocking diode. Capacitors C1 acts asstorage devices for the approximate 26 VDC output by diode CR1.Resistors R2 and R3 function as a voltage divider to set the output ofpower supply U1 to 7 VDC. Diode CR13 in combination with capacitors C15and C17 function to set the output of power supply U2 to 5 VDC.

Returning to FIG. 25, the power supply ground fault protection isaccomplished by monitoring the current of the 28 VDC power and loadreturn signals with the imbalance sensor 44. This current sensor 44algebraically sums the magnetic flux generated by the power and returncurrents and produces an output signal 42 that is proportional to theresult. With the 28 VDC power and load return signals of the fuel pumpspassing through the ground fault sensor, the system is “closed”,requiring the current going to the fuel pump to be equal and oppositethe return current. Therefore, when a ground fault condition does notexist, the magnetic flux measured at the sensor is zero. When a groundfault condition occurs, current flows to ground (which does not returnthrough the sensor), breaking the closed loop system and resulting in amagnetic flux imbalance measured at the sensor. Since the flux imbalanceis proportional to the current, the output 42 of the sensor provides themagnitude of the current loss.

Although the ground fault nomenclature implies an arc/short to ground,the detection method senses a current imbalance condition due to currentloss as a result of a power to ground path or from power to any otheraircraft wiring outside the closed loop system. The system thus providesdetection of any incorrect connection downstream of its installation.

A 28 VDC to load-return arc/short would not be detected as ground faultsince a 28V to load-return current flow is normal and the closed loopsystem does not result in current loss. Although this arc-shortincreases the current amplitude beyond normal levels, this is not animbalance condition and the net corresponding magnetic flux sum at theground fault sensor will be zero.

In order to detect this undesirable fault condition, over-currentprotection is achieved with the over-current sensor 46 to monitor thecurrent level of the 28 VDC power signal. The magnetic flux generated bythe current passing through this sensor 46 produces an output signalproportional to current amplitude. An over-current condition exists whenthe current amplitude exceeds a predetermined threshold limit. Similarto ground fault detection, over-current detection latches a trippedcondition. The threshold limit for over-current detection is chosen witha sufficient margin to prevent the fuel pump maximum startup and pumpmaximum operating currents from creating nuisance trips. Due to thedifferences in the startup and operating currents, two threshold limitsare used for the over-current design, in a manner similar to thatpreviously discussed with respect to other embodiments of the invention.

In operation, the system monitors the electrical circuit 20 between theline side 24 and the load side 26 for undesirable ground faults andover-current conditions. These flux signals produced by the sensors 44,46 are input to the logic controller 50. The logic controller 50conditions and passes the signals through filters to eliminate noisefluxes of either polarity. When a ground fault or over-current conditionoccurs, the logic controller 50 interrupts the return path 34 of therelay power input 32. This causes the relay 60 to latch open therebyinterrupting the current path 20 to the load side 26. An LED 52indicates the detected fault. The relay 60 remains latched until resetmanually by pressing the reset button 54 on the system housing. Aseparate latching relay (described further below) in the logiccontroller 60 is used to store the fault condition resulting inindefinite retention of the fault condition. Thus, cycling the fuel pump28 VDC power via the control switch 82 does not reset the system. Whenthe system is not in a faulted state, control of the fuel pump isachieved through the aircraft pump control switch 82.

The manual reset switch 54 prevents aircraft cockpit cycling of switch82 and circuit breaker 84 from reapplying power back into an existingground fault or over-current condition. A manual reset forces aconscious maintenance action that should be performed after performingthe necessary maintenance troubleshooting to fault isolate the failure.

A periodic maintenance inspection to verify the integrity of the systemis performed by pressing two press-to-test switches located on thehousing. One switch provides a ground-fault test 56 and the otherprovides an over-current test 58. Positive visual indication to show theFBP is functioning properly is provided by the LED 52.

In one configuration of the standalone current-fault protection device,a ground fault detection is accomplished when a current imbalance existsthat is outside the acceptable range of −1.5 A RMS to +1.5 A RMS. Thisrange provides sufficient margin to prevent leakage current thatnormally occurs with components submerged in fuel from causing nuisancetrips.

Over-current detection is achieved with a separate sensor to monitor thecurrent level of the 28 VDC power. The magnetic flux generated by the 28VDC current passing through this sensor produces an output signalproportional to current amplitude, resulting in an over-currentcondition when the current amplitude exceeds the predeterminedthreshold. Similar to the ground fault condition, over-current detectionlatches a tripped condition. As with earlier described embodiments, thethreshold limit for over-current detection is chosen with sufficientmargins to prevent normal startup and operating conditions from creatingnuisance trips. Thus, the over-current design has a two-tier threshold,one threshold during startup and another threshold for operation,similar to those previously described with reference to otherembodiments of the invention.

Pressing the ground fault test switch 56 simulates a ground faultcondition by passing a current through the imbalance sensor 44 that isjust above the maximum threshold. The manual test provides visualindication of the ground fault detection by lighting the LED 52,latching the fault condition and opening the relay 60. The reset switchmust be pressed to clear the latched ground fault condition, LED, andallow the relay 60 to close. The over-current test switch 58 simulatesan over-current condition by passing a current through the over-currentsensor 44 that is just above the threshold. Visual indication of theover-current detection is provided by lighting the LED 52, latching thefault condition and opening the relay 60. Pressing the reset switch 54again clears the device and completes the end-to-end test, rearming thedevice for normal use. When performing the end-to-end test, the groundfault test switch 56 and over-current test switch 58 can be pressed ineither order. The switches are guarded to prevent accidental pressing.In an alternate configuration, the system is provided with two LEDs, onefor providing an indication of a ground fault condition and the otherfor providing an indication of an over-current condition.

In the event of a real fault condition, the device de-energizes itsrelay and removes power to the pump. The device does not have any directoutput to the aircraft alerting system, so the flight crew is alerted tothe fault condition by the existing alerting method of the componentbeing protected. When the motor is de-energized the pump loses pressureand the boost pump pressure switch alerts the flight crew that the boostpump is inoperative. Any attempts to turn on the boost pump again haveno results. Resetting the device after a fault condition is accomplishedby ground maintenance crew after performing the necessary faultisolation procedures.

With reference to FIGS. 27 a through 27 c, the logic controller includescircuitry similar to that previously described with reference to otherembodiments of the invention. Specifically, the sensor control circuitry(FIG. 27 a) includes an amplifier U3A that conditions the imbalancesignal received from the imbalance sensor and amplifier U4A thatconditions the over-current signal received from the over-currentsensor. The output of the over-current sensor is input to amplifiers U4Band U4C. Component U6, in combination with transistor Q7 and resistorsR19 and R20, set the over-current threshold to either a normal operationthreshold or a start-up threshold in a manner similar to that describedwith reference to FIG. 8 a.

The maintenance circuitry (FIG. 27 c) includes three switches S1, S2 andS3. Switch S1 functions as a reset switch like that previously describedwith reference to FIG. 5 b. Switch S2, resistor R41 and coil L1 functionas the press-to-test circuit with respect to the imbalance sensor whileswitch S3, resistor R42 and coil L2 function as the press-to-testcircuit with respect to the over-current sensor. In each case the coilis wrapped around its associated current sensor a sufficient number oftimes such that when its associated switch is closed the sensor outputsa signal indicative of a current fault condition. The rest of the logiccircuitry, as shown in FIGS. 27 b and 27 c, is similar to thatpreviously described with reference to FIGS. 5 b and 5 c-4.

AC Configuration

With reference to FIG. 28, in another configuration the system isadapted to monitor the current path between an AC power source and aload powered by the power source. The system includes a power supply 30that taps off of each of the 115 VAC three phase lines at the input side24 of the electrical circuit. The power supply 30 provides power to thesensor system 40, the logic controller 50 and the power controller 60.The sensor system 40 includes a single sensor 44 and three over-currentsensors 46. The single sensor 44 determines the current condition in thecurrent path 20 by providing an output sensor signal 42 indicative ofthe current balance among the electrical lines. Each of the over-currentsensors 46 outputs a signal 48 indicative of the amount of currentpassing through its associated electrical line.

These signals are provided to the logic controller 50 where they arecompared against maximum acceptable threshold values in a manner similarto that described with reference to FIGS. 8 a-1 through 8 c-4. If any ofthe sensor signals 42, 48 does not satisfy the preestablished criteria,the return path 34 of the power supply 32 to the AC relay 60 isinterrupted.

Solid State Configuration

With reference to FIG. 29, in another configuration the system isadapted to monitor the current path between an AC power source and aload powered by the power source. The system includes a power supply 30that taps off of each of the 115 VAC three phase lines at the input side24 of the electrical circuit. The power supply 30 provides power to thesensor system 40, the logic controller 50 and the power controller 60.The sensor system 40 includes a single sensor 44 and three over-currentsensors 46. The single sensor 44 determines the current condition in thecurrent path 20 by providing an output sensor signal 42 indicative ofthe current balance among the electrical lines. Each of the over-currentsensors 46 outputs a signal 48 indicative of the amount of currentpassing through its associated electrical line.

These signals are provided to the logic controller 50 where they arecompared against maximum acceptable threshold values in a manner similarto that described with reference to FIGS. 8 a-1 through 8 c-4. If any ofthe sensor signals 42, 48 does not satisfy the preestablished criteria,the logic controller 50 switches the solid state device to an openposition, thereby interrupting the current path to the load side 26.

Packaging

With reference to FIGS. 30 a-30 f, the above-described standalonecurrent fault protection devices can be contained in a correspondinghousing 200 which in one embodiment has the following dimensions:approximately 2.50 inches (about 6.35 cm.) from top 202 to bottom 204and approximately 4.00 inches (about 10.16 cm.) along its sides 206. Thehousing also includes a line input connector 208 and a line outputconnector 210 for interfacing with existing aircraft wiring.

Referring to FIGS. 25 and 30 d, the input connector 208 accommodates aDC input line from the aircraft while the output connector 210accommodates a DC line to the aircraft load. The circuitry forming thepower supply 30, the logic controller 50 and the maintenance system 70is mounted to a circuit board 212. The maintenance system 70 circuitryinterfaces with the push/pull button 186 and the Test button located onthe top of the housing 180. The sensor system (not visible) arepositioned beneath the circuit board 212. The input lines A and B passthrough the sensor system power controller 60 and are input to the powercontroller 60.

From the above, it may be seen that the present invention provides amethod and apparatus for maintaining the current conditions inelectrical equipment in aircraft which may be adapted to a variety ofsystems and components. As such, it provides additional reliable andrapid disconnect of power to the existing systems, thus reducing damagefrom ground faults or over-current conditions in the circuits. While aparticular form of the invention has been illustrated and described itwill also be apparent that various modifications can be made withoutdeparting from the spirit and scope of the invention. Accordingly, it isnot intended that the invention be limited except as by the appendedclaims.

1-55. (canceled)
 56. A current fault detector and circuit interrupterfor retrofitting a circuit breaker in an existing aircraft applicationwherein such circuit breaker complies with a form factor, engages aconnector to interface with a line side and a load side of an electricalcircuit and controls a current path from said line side to said loadside, comprising: a power controller adapted to close the current pathin the presence of a control signal and open the current path in theabsence of the control signal; terminals for engaging said connector; asensor system adapted to monitor the current in the current path andoutput a sensor signal indicative of a current condition within thecurrent path; a logic controller adapted to receive the sensor signaland remove the control signal from the power controller when the sensorsignal does not satisfy an established criteria; and a housing forcontaining at least said power controller, said sensor system and saidlogic controller, wherein said housing complies with said form factor.57. The device of claim 56 wherein the current path comprises threeelectrical lines, the sensor system comprises a single sensor associatedwith the electrical lines and the current condition comprises thecurrent balance among the electrical lines.
 58. The device of claim 56wherein the current path comprises three electrical lines, the sensorsystem comprises three sensors, each associated with one of theelectrical lines and the current condition comprises the currentmeasurement in each electrical line.
 59. The device of claim 56 whereinthe current path comprises three electrical lines, the sensor systemcomprises four sensors, three sensors each associated with one of theelectrical lines individually and one sensor associated with the threeelectrical lines collectively, and the current condition comprises thecurrent balance among the electrical lines and the current measurementin each electrical line.
 60. The device of claim 56 wherein the currentpath comprises a DC voltage line and a return line, the sensor systemcomprises a single sensor associated with the lines and the currentcondition comprises the current balance among the DC voltage line andthe return line.
 61. The device of claim 56 wherein the current pathcomprises a DC voltage line and a return line, the sensor systemcomprises a single sensor associated with the DC voltage line and thecurrent condition comprises the current measurement in the DC voltageline.
 62. The device of claim 56 wherein the current path comprises a DCvoltage line and a return line, the sensor system comprises two sensors,one sensor associated with the DC voltage line individually and onesensor associated with the DC voltage line and the return linecollectively, and the current condition comprises the current balanceamong the DC voltage line and the return line and the currentmeasurement in the DC voltage line.
 63. The device of claim 56 whereinthe sensors comprise any one of a Hall effect sensor, a currenttransformer and a giant magneto resistive sensor.
 64. The device ofclaim 56 wherein the logic controller comprises electrical circuitryconfigured to compare the sensor signal to a preestablished thresholdrange and to output a fault signal when the sensor signal is not withinthe range.
 65. The device of claim 64 wherein the sensor systemcomprises a sensor associated with at least two electrical lines, thesensor adapted to output a sensor signal indicative of the currentbalance among the electrical lines and the predetermined threshold rangeis between approximately −1.5 A RMS and +1.5 A RMS.
 66. The device ofclaim 64 wherein the load side of the electrical circuit is connected toan electrical load having an associated operating current, the sensorsystem comprises at least one sensor associated with one electricalline, the sensor adapted to output a signal indicative of the currentpassing through the electrical line and the predetermined thresholdvalue is approximately 1.25× the operating current.
 67. The device ofclaim 64 wherein the load side of the electrical circuit is connected toan electrical load having an associated first operating current for afirst amount of time and a second operating current for a second amountof time and the logic controller comprises circuitry adapted to set thepredetermined threshold value at a first level during the first amountof time and a second level during the second amount of time.
 68. Thedevice of claim 64 wherein the first amount of time comprises thestartup time associated with the electrical load and the second amountof time comprises time other than the startup time.
 69. The device ofclaim 56 wherein the power controller comprises a DC relay.
 70. Thedevice of claim 56 wherein the power controller comprises a AC relay.71. The device of claim 56 wherein the logic controller is furtheradapted to monitor an external on/off power switch and remove thecontrol supply from the power controller when either of the followingconditions occur: the sensor signal does not satisfy an establishedcriteria, or the external power switch is off.
 72. The device of claim56 further comprising test circuitry adapted to cause the sensor systemto output a sensor signal that does not satisfy the establishedcriteria.
 73. The device of claim 56, wherein said power controller,sensor system and logic controller are packaged within a single housing.74. The device of claim 73, further comprising a light emitting diodeinterfacing with the logic controller, positioned on the outside of saidhousing and adapted to illuminate when the sensor signal does notsatisfy the established criteria.
 75. The device of claim 73, furthercomprising a mechanical indicator having first and second positions,interfacing with the logic controller and positioned on the outside ofsaid housing and adapted to change position when the sensor signal doesnot satisfy the established criteria.
 76. The device of claim 73,further comprising buttons positioned on the outside of said housing fortesting and resetting said device.